1. Field of the Invention
The present invention relates to an organic electro luminescence device and fabrication method thereof, and more particularly, to an organic electro luminescence device in which an amorphous silicon thin film transistor is employed as a driving element and a fabrication method thereof.
2. Discussion of the Related Art
A liquid crystal display device (LCD) having advantages of lightweight, slim profile and low power consumption has been the highlight of flat panel displays attracting public attention.
However, since the LCD is a passive device, not a light emitting device (i.e., active device), it has technical limitations in brightness, contrast, viewing angle, large-sized screen and the like. To this end, new flat panel displays capable of overcoming the disadvantages of the LCD are being actively researched.
Among such flat panel displays, an organic electro luminescence device (ELD) is a self-emission type display that has a high contrast and a wider viewing angle. The organic ELD can be made having lightweight and slim profile compared with other displays because it does not require a backlight. It also can decrease power consumption compared to other displays.
Further, the organic ELD can be driven with a low DC voltage at a rapid response rate. Since all of the components of the organic ELD are formed of solid materials, it is durable against external impact. It can be also used in a wide temperature range and be manufacture with low cost.
In particular, since the organic ELD is manufactured through only a deposition process and an encapsulation process, the manufacturing process and apparatus are very simple, which is unlike the method for fabricating an LCD or a plasma display panel (PDP).
Also, in driving the ELD in the active matrix way that each pixel has a thin film transistor functioning as a switching element, even when a low current is applied, uniform luminescence can be obtained. As a result, the organic ELD has advantages of low power consumption, high definition and large-sized screen.
Such an active matrix type organic electro luminescence device (hereinafter referred to as ‘AMOLED) will be described hereinafter with reference to the accompanying drawing.
FIG. 1 shows a circuit diagram illustrating a basic pixel structure of a related art AMOLED.
As illustrated in the drawing, gate lines GL 2 are formed in a first direction and data and power lines DL 3 and VDD 4 are formed in a second direction intersecting the first direction to define a unit pixel region.
A switching TFT 5 that is an addressing element is formed on each crossing point of the gate and data lines 2 and 3. A storage capacitor CS 6 is connected with the switching TFT 5 and the power line 4. A drive TFT 7 that is a current source element is connected with the storage capacitor CS 6 and the power line 4. An organic electro luminescence diode 8 is also connected with the drive TFT 7.
When current is applied to the organic light emitting material in a forward direction, electrons and holes are recombined, moving through a P-N junction between an anode electrode as a hole donor and a cathode electrode as an electron donor. Therefore, the energy of the organic electro luminescence diode 8 becomes lower than that created when the electrons are separated from the holes. At this point, energy difference is generated, thereby emitting light.
In other words, the unit pixel of the AMOLED essentially includes the switching TFT 5 for addressing a pixel voltage that is a gate driving voltage, a drive TFT 7 for controlling a drive current of the AMOLED, and a storage capacitor 6 for stably maintaining a pixel voltage.
The organic electro luminescence device can be classified into a top emission type and a bottom emission type according to an advancing direction of the light emitted from the organic electro luminescence diode.
The TFT used in the AMOLED can be classified into an amorphous silicon (a-Si) TFT and a polycrystalline silicon (p-Si) TFT according to states of a thin semiconductor film functioning as an active channel.
Recently, research to employ the p-Si TFT having a high field effect mobility in the AMOLED has been actively performed, but it is more typical to employ the a-Si TFT in the AMOLED.
FIG. 1 illustrates the AMOLED employing the a-Si TFTs. The a-Si TFTs are an n-type a-Si TFT. Accordingly, as illustrated in FIG. 1, the AMOLED is connected to the source electrode (S) of the drive TFT 7, and the power line 4 is connected to the drain electrode D of the drive TFT 7.
FIG. 2 is a schematic sectional view illustrating a related art bottom emission type AMOLED.
As illustrated in the drawing, the bottom emission type AMOLED includes a first transparent substrate 12, an array part 14 formed on the first transparent substrate 12, and an anode 16, an organic emission layer 18 and a cathode sequentially formed on the array part 14 and forming an organic electro luminescence diode.
At this point, the organic emission layer 18 represents red R, green G and blue B colors. For example, organic material emitting R, G and B colors are patterned on each pixel P. Alternatively, the organic emission layer 18 may be made in a multi-layer structure formed of organic material.
In other words, the organic electro luminescence layer 18 can be formed between the anode and the cathode by sequentially depositing a hole injection layer (HIL), a hole transporting layer (HTL), an emission layer (EML), and an electron transporting layer (ETL).
The first substrate 12 is attached with the second substrate 28, on which an absorbent 22 is formed, by a sealant 26, thereby completing the organic electro luminescence device that is encapsulated.
The absorbent 22 is for removing moisture and oxygen that may infiltrate into the encapsulated organic electro luminescence device. A portion of the substrate 28 is etched, and the absorbent 22 is filled in the etched portion and fixed by a tape.
FIG. 3 is a sectional view partially illustrating a TFT array part of the related art AMOLED depicted in FIGS. 1 and 2. Specifically, FIG. 3 illustrates a section of a region including a drive TFT of the TFT array part.
Generally, in an AMOLED, each of the pixels of the TFT array formed on the substrate is provided with a switching element, a drive element and a storage capacitor. Depending on the operational characteristics, the switching element or drive element can be formed of a combination of more than one TFT.
Each of the switching TFT T and the drive TFT TD includes a gate electrode, an active layer, a source electrode and a drain electrode. At this time, the TFTs used in the AMOLED can be classified into a-Si TFTs and p-Si TFTs according to states of a thin semiconductor film functioning as an active channel.
FIG. 3 illustrates the AMOLED employing the a-Si TFTs. At this time, the a-Si TFTs are n-type a-Si TFTs as aforementioned. Accordingly, the anode of the AMOLED is connected to the source electrode (S) of the drive TFT.
Referring to FIG. 3, the drive TFT TD includes a gate electrode 30, a gate insulating layer 31, a source electrode 33, and a drain electrode 34. An active layer 32 is disposed between the source electrode 33 and the drain electrode 34.
Also, a pixel region is configured to include an anode 36 connected with the source electrode 33, an organic emission layer 38 formed on the anode 36 in a single layer structure or a multi-layer structure, and a cathode 39 formed on the organic emission layer 38, for injecting electrons. The anode 36 injects holes into the organic emission layer 38.
The organic emission layer 38 having a multi-layer structure can be configured to include the HIL, HTL, EML and ETL, as aforementioned.
The pixel regions are arranged in a matrix configuration and are separated from one another by a buffer 37.
In other words, the related art AMOLED is configured to include the drive TFT TD formed on the pixel region; the anode 36 connected with the source electrode 33 and functioning as a pixel electrode; the buffer 37 formed on the anode 36 for partitioning the pixel regions from one another; the organic emission layer 38 comprised of the HIL, the HTL, the EML and the ETL within the buffer 37; and the cathode 39 formed on the organic emission layer 38.
From the drawings of FIGS. 1 through 3, it is well known that the related art AMOLED employing the a-Si TFT as a drive TFT is configured to include the anode 36 connected with the source electrode 33 of the drive TFT TD, and the organic emission layer 38 and the cathode 39 disposed on the anode 36.
In other words, according to the construction of the related AMOLED, the anode 36 connected with the source electrode 33 of the drive TFT TD functions as a pixel electrode, and the cathode 39 functions as a counterpart electrode, i.e., a common electrode, which is contrary to the general structure where the cathode functions as the pixel electrode and the anode functions as the common electrode.
Hence, when the pixels of the AMOLED are configured in the above construction, the circuit is not stable so that a driving failure may be caused.